The get-togethers — which have been held in New York and Los Angeles and are planned for other cities — ask guests to submit a slept-in T-shirt that will be smelled by other participants.

Then, voila! You can pick your partner based on scent, or so the theory goes.

The parties started out as an experimental matchmaking fest by a California woman weary of online dating, but it turns out they also have a root in science. Researchers have shown that humans can use scent to sort out genetic combinations that could lead to weaker offspring.

At a dimly lit art gallery in Los Angeles on a recent night, partygoers huddled around several tables covered with plastic freezer bags stuffed with shirts and an index card bearing a number. Once they found one they liked, a photographer snapped a picture of them holding the bag and projected it onto a wall so the shirt's rightful owner could step forward and meet his or her odor's admirer.

Konstantin Bakhurin, a 25-year-old neuroscience graduate student, said he bypassed the bags that smelled like baby powder or laundry detergent or perfume in search of something more unique: the owner of a distinctive yellow-T-shirt whose fragrance he described as "spicy."

"I think it's probably a bit more pseudoscience," said Bakhurin, who attended with two fellow graduate students from University of California, Los Angeles. "I just kind of came here for kicks to see what would happen."

The parties are a marked contrast to the proliferation of online dating sites, which demand countless details from singles, and in some ways are taking romance back to its most primal beginnings.

Judith Prays, a web developer who now lives in Atlanta, said she came up with the idea for pheromone parties after she failed to find a match online. Prays said she'd date men for a month or so before things soured until she started seeing a man who wasn't what she was looking for and wound up in a two-year relationship.

What she remembered was his smell.

"Even when he smelled objectively bad, I thought he smelled really good," the 25-year-old said. "And so I thought, OK, maybe I should be dating based on smell?"

At first, it was an experiment. Prays invited 40 friends to a party in New York and asked them to sleep in a T-shirt for three nights, put it in a plastic bag and freeze it, then bring it to the party. Bags were coded with blue cards for men and pink for women and numbered so the shirts' owners could pinpoint their admirers.

The night was a hit, Prays said, adding that half a dozen couples hooked up and one pair formed a relationship. Since then, she has held similar parties in New York and Los Angeles and is planning others for Atlanta, San Francisco and perhaps elsewhere.

Many partygoers chuckled at the idea of finding a match in a smelly T-shirt. But that's not to say there isn't some science supporting the idea.

Research studies using similar T-shirt experiments have shown that people prefer different human scents. But whose smell they prefer is dictated by a set of genes that influence our immune response — which researchers say is nature's way of preventing inbreeding and preserving genetic adaptations developed over time.

"Humans can pick up this incredibly small chemical difference with their noses," said Martha McClintock, founder of the Institute for Mind and Biology at the University of Chicago. "It is like an initial screen."

In one such study, McClintock and her colleagues had participants sniff inside a covered box without knowing that in some cases they were smelling worn T-shirts. What they found was people preferred the odors of those who had different genetic makeups from their own, but not radically different.

In Los Angeles, several dozen 20-somethings headed to the gallery at night in search of romance — or at least out of curiosity. They posed playfully for the photographer with shirts they liked, hoping the owner might step forward and say hello.

Few did. Some admitted they had seen their number flashed on the wall but were too shy to identify themselves.

But there was still plenty of chatter as beer-sipping singles turned up their noses at bags that smelled like hiker's sweat and their aunt's old carpet and took a second whiff of sweet and musky fragrances they liked but couldn't peg — a playful exercise that served as an icebreaker to what otherwise could have been an awkward gathering of strangers.

Karen Arellano threw back her head and laughed after trying a handful of bags that reeked of sweat, coffee and even weed — but said she didn't really come to the party in search of love.

Prays said she's also learned from the experience that while scent is powerful, it isn't enough to detect a good match.

"Animals have babies and they move on, and that's what the pheromone party is," said Prays, who may start including a few pertinent details on the index cards, like a person's relationship expectations. "The most successful thing about it is it opens up conversation."